Huygens' wave theory of light has become more or less high school curriculum, but you should realize that his model is purely a mathematical and mechanical model and therefore not necessarily in accordance with reality. His contemporaries also expressed a number of objections:
Why is the new wave front formed by the tangent line to the elemental waves?
What happens to the parts of those circularly expanding elemental waves which do not participate in the new wave front?
Why don't the backwards running elemental waves create backward running wave fronts?
How do circular elemental waves explain the observed linear propagation of light?
What is it that is oscillating, Christiaan?
So, the end of Newton's corpuscle model was still a long way off. The theory of light corpuscles would last until 1803 when Thomas Young, through interference experiments with sunlight, demonstrated the wave character of light convincingly and presented his results to the Royal Society in London.
3: The clockwork universe and the ether
""A scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up that is familiar with it." ~ Max Planck."
Max Planck, first quantum physicist 1858-1947
In this chapter you will be introduced to the way how light was recognized as a wave phenomenon by the discovery of interference effects. This was one of the first moments in history where Newton's ideas of the universe were severely contradicted. The acceptance of the idea of the wave character of light gave rise to speculations about in what substance it would be oscillating. An intangible ether was assumed initially. Electromagnetic effects were investigated and exactly measured in the first part of the 19th century. Electric and magnetic fields were introduced as mathematical abstractions and acquired quickly the status of objective existing phenomena.
Electromagnetic waves traveling with the velocity of light were predicted by Maxwell's theory and were soon confirmed by experiment. Sophisticated attempts to measure the speed at which the earth would travel through the assumed ether failed utterly and confirmed thereby the predicted constancy of the speed of light regardless of the position and the movement of the observer through the supposed ether.
At the brink of the twentieth century Max Planck solved the enigma regarding the radiation of a hot body by assuming exchange of electromagnetic energy in discrete amounts - quanta. His solution spelled the ultimate doom for the Newtonian vision of a 100% material universe.
Light as a wave phenomenon, interference, superposition
Thomas Young (1773-1829) let sunlight light shine through a double-slit, set in the one of the sides of a closed box. The box had a piece of frosted glass at the opposite side in order to observe the pattern that was projected by the double-slit. He created the double-slit simply by making two narrow parallel scratches on a soot-coated piece of glass. A pattern of colored light and dark bands appeared in Young's experiment on the frosted glass at the back of the box. A phenomenon that could not be explained by Newton's concept of light corpuscles. This could only be explained by assuming that light behaved as waves.
Figure 3.1: Youngs drawing of interference of monochrome light waves.
In 1803 Young presented his explanation of the results of his double-slit experiment to the Royal Society in London. He produced the above sketch to explain these dark and light bands. The cause of this phenomenon is called interference. Light waves, coming from the left - not shown here - will arrive at the narrow slits A and B. In these slits two synchronously vibrating elementary wave sources are created, in the way Huygens had already supposed. The two synchronous wave sources will generate circular wave fronts expanding from both slits. These synchronous circular expanding waves are necessarily moving exactly in phase and will have the same wavelength. Young's sketch - figure 3.1 - represents the circular crests - the maximum height - of the supposed waves. Where these wave crests, expanding from slits A and B, intersect, they reinforce each other and create a higher wave crest, a maximum. Where the wave troughs - the minimum heights - extending from A meet the wave crests from B - and vice versa for B and A - they will annihilate each other.
In Young's sketch you can clearly see that the maxima will be found along the lines fanning out from between the slits. These lines are formed by connecting the intersecting wave crests. Where those lines of maximum wave crests reach the screen (C-D-E-F), the light will show maximal intensity. Halfway in between these maximum wave crests the light waves will annihilate each other so you will observe darkness where they reach the screen. This reinforcing and annihilating phenomenon is called interference.
Wikipedia: In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive interference result from the interaction of waves that are correlated or coherent [1] with each other, either because they come from the same source or because they have the same or nearly the same frequency [2]. Interference effects can be observed with all types of waves, for example, light, radio, acoustic, surface water waves, gravity waves, or matter waves [3]. The resulting images or graphs are called interferograms.
The moiré effect [4] - see figure 3.2 - is also an interference phenomenon. When you slide two sets of concentric circles, drawn on transparent material, over each other the visual result is utterly similar to Young's drawing in figure 3.1. Watch this in the moiré animation film by Amanita [5]. Where the circles overlap the interference is constructive. Observe how the curves of constructive interference move depending on the varying distance of the two central circles that can be equated to the two wave sources in the slits.
Figure 3.2: Moiré effect of two overlapping sets of concentric circles. Compare this with Young's sketch of wave interference.
Source: Wikimedia Commons.
Frequency - usually denoted by the Latin letter f or the Greek letter ν (pronounced 'nu') - tells us the number of complete vibrations per second. The international standard unit for frequency is the Hertz (Hz), which is the number of oscillations per second. The most elementary wave type used in physics is the sine wave. The phase of a sine wave is expressed in degrees of a circle and represents the state of the wave within the timespan of a full oscillation period. So:
0o is the moment when the wave height is zero and rising,
90o is the moment when the wave height is maximal (a crest),
180o is the moment when the wave height is zero and falling,
270o is the situation when the wave height is minimal (a trough).
When two waves are in opposite phase, their phase difference is 180o.The value of the deflection from the middle where the maximal height is reached (90o)